The scattering function was determined for four samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMMA), each with the fraction of racemic diads f(r) similar or equal to 0.01, in the range of weight-average molecular weight M(w) from 1.01 X 10(3) to 9.87 X 10(4) in acetonitrile at 28.0 degrees C (theta) for the magnitude k of the scattering vector smaller than 1 Angstrom(-1) by the use of a point-focusing small-angle X-ray scattering (SAXS) camera. The Kratky function F-s as a function of k increases monotonically with increasing k irrespective of the values of M(w) and does not exhibit a maximum followed by a minimum, as observed for atactic (a-) or syndiotactic (s-) PMMA. It is shown that the helical wormlike (HW) chain theory may explain rather well the present SAXS data over the whole range of k studied. However, the agreement between theory and experiment in the range of large k may be only accidental. A rather detailed comparison is made of the present data for i-PMMA with the previous data for a-PMMA with f(r) = 0.79 and s-PMMA with f(r) = 0.92 in acetonitrile at 44.0 degrees C (theta), and the dependence of F-s on f(r) is discussed, it being consistent with the HW theory. A comparison is also made of the present SAXS data with literature data obtained for i-PMMA in the bulk from small-angle neutron scattering (SANS) measurements. As in the case of a-PMMA, it is concluded that the difference between the Kratky plots (F-s) of the two kinds of data in the range of large k arises from the difference in the distribution of scatterers between SAXS and SANS.